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Changes in Chemical Composition of Kentucky Bluegrass and Tall Fescue Herbages Following N Fertilization¹

Most research on N fertilization of grasslands has dealt with yield responses, with much less attention to feed quality. These field studies were undertaken to determine the effects of four rates of N and harvest time on levels of crude protein (CP), P, Ca, K, and Mg in spring and early-summer aftermaths of Kentucky blue-grass (Poa protensis L.) and tall fescue (Festuca arundinacea Schreb.) grown on Typic Paleudults soil.

NH₄NO₃ was applied to supply 50, 100, 150, and 200 kg N/ha. The fertilizer treatments were replicated in time (9 April, 30 April, and 21 May), as well as space, to introduce different environmental conditions and plant development stages. Herbage samples were collected weekly during three 6-week periods following fertilization.

CP in both grasses increased with larger applications of N, reaching peak values of 32.8 and 32.5% in bluegrass and tall fescue, respectively, in the first growth period., 2 weeks after fertilization. Minimum values of 11.8 and 9.7% occurred in 6-week growths of bluegrass and fescue, respectively, with 50 kg N/ha.

Effects of N on P concentrations were inconsistent. Concentrations of Ca and K in both grasses rose with increasing N, as did Mg in the first two growth periods. In the second half of period 3, high N decreased Mg concentrations. The latter effect and the fact that high levels of N reduce Mg availability to animals indicate an increased likelihood of tetany, especially with bluegrass, when large amounts of N are applied in spring.

Elemental concentrations of Ca, P, and K in both grasses appeared adequate to meet ruminant requirements, irrespective of N application. With the possible exception of K, none of the four elements appeared to be limiting for plant growth. Bluegrass herbage, especially in growth period 3, often contained less than 2.0% k.

Ca/P ratios increased with higher applications of N in periods 2 and 3, but not in 1. Effects of N on K/(Ca+Mg) ratios were inconsistent, but values only once reached the critical level of 2.2.

Key Words: Growth period • Ca/P ratio • K/ (Ca+Mg) ratio • Ca • Mg • P • K • Grass nutritive value

² Associate professor, Soils Dep., Pahlavi Univ., Shiraz, Iran (former visiting associate professor, Dep. of Agronomy, Univ. of Kentucky), and professors of agronomy, Univ. of Kentucky, Lexington, KY 40506.

Received for publication June 18, 1976.

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